Age of the Zhongtiaoshan Dyke Swarm: Constraints on the Nature of the 1.8 Ga Large Igneous Province Event in North China

The geodynamic regimes that led to the formation of the early Mesoproterozoic (1.80-1.76 Ga) large-scale mafic dyke swarm in the North China Craton (NCC) have long been controversial. One school of scholars insisted their preference for post-collisional lithospheric extension related to amalgamation of the Eastern and Western blocks, while others argue for a mantle plume-triggered intracontinental rift setting. This owns much to the failure to characterize the refined temporal-spatial distribution of the dyke swarm. In the Zhongtiaoshan area, numerous low-grade metamorphosed to unmetamorphosed intermediate-mafic dykes constitute an important portion of the southern segment of this giant dyke swarm. Nevertheless, precise chronological constraints for their emplacement timing are still lacking. Given the superiorities of the baddeleyite U-Pb isotope system in dating silica-undersaturated igneous rocks, we plan to determine the formation age of dykes in the southern Zhongtiao Mountain by in-situ Pb/Pb dating of baddeleyites using an ion microprobe. Combined with a brief introduction to the field geologic and geochemical characteristics of the Zhongtiaoshan dykes, this paper is intended to provide new clues to understanding the deep dynamic process of magmatism represented by the dyke swarm. We sampled an EW-trending dyke near Wangzhuang Village in Yongji City. A baddeleyite Pb-207/Pb-206 dating for this sample yields a magmatic crystallization age of 1799 +/- 4 Ma (n=20, MSWD=2.2). This result effectively fills the vacancy of dyke ages in the key area, and reinforces the view that the early Mesoproterozoic complex dyke swarm comprises two stages of dykes, namely, the Xiong'er (1800-1785 Ma) and Taihang (1770-1760 Ma) dyke swarms, which are exposed in the middle-southern portion of the central NCC and throughout the central NCC, respectively. Meanwhile, we also analyzed the whole-rock elemental and Nd isotopic compositions of the Yongji dyke from the Zhongtiaoshan area. It reveals that the trace element distribution patterns and the Nd isotope composition of the ca. 1799 Ma Zhongtiaoshan dyke are similar to those of the Xiong'er volcanic rocks and its coeval dyke swarm, indicating that they belong to the same tectonic-thermal event and share a common magma origin. As the early generation of the early Mesoproterozoic large igneous provinces, the 1800-1785 Ma Xiong'er dyke swarm of approximately E-W trends seems difficult to match the post-collisional extension stress field of the SN-trending trans-North China orogen, while a series of arc-like or linear faults formed around a mantle plume during the stress-changing process from domal uplift to the onset of rifting and break-up can provide a suitable emplacement environment for these dykes. On the other hand, the spatial migration trend of the Xiong'er and Taihang dyke swarms as a whole also does not conform to the feature of a post-orogenic dyke swarm. The constituted members of the latter are generally coeval along the orogenic belt. In such a situation, the young orogenic belt of weakness in the cratonic basement had only controlled the localization of dyke swarms on a macroscopic scale. Therefore, we support that the Xiong'er and Taihang dyke swarms are the products of multi-stage interaction between a mantle plume and the overlying and moving continental lithosphere of the North China Craton. In addition, the ca. 1799 Ma Zhongtiaoshan dyke located close to the plume-sourced magma center is the oldest unmetamorphosed dyke reported from the North China Craton, which gives an insight to constraining the lower boundary of the Xiong'er Group by revising the initial emplacement time of the Xiong'er dyke swarm, and affords new evidence for terrane correlations and supercontinent reconstructions based on the occurrence of large igneous provinces or dyke swarms.